Method and system for selecting a best serving sector in a CDMA data communication system

ABSTRACT

In a disclosed embodiment, signal levels of the active sectors of an access terminal are compared with the signal level of the current serving sector of the access terminal. Next, a delta credit is accumulated. If a DRC lock bit is available, then an accumulated total credit is authorized to produce an authorized accumulated total credit. Afterwards, a new serving sector is identified from a pool of candidate sectors based on the signal levels of the active sectors and the authorized accumulated total credits.

BACKGROUND

1. Field

The present invention generally relates to wireless communicationsystems and more particularly to a method and system for site selectiontransmit diversity (SSTD) in a CDMA data communication system.

2. Related Art

The current generation of cellular phone systems offers more servicesthan those of previous generations, such as data services. First andsecond generation cellular communication systems were typically usedmostly for voice services. Second generation systems began addinglimited data services, albeit at low data rates. Third generationsystems, such as the Code Division Multiple Access (“CDMA”) High DataRate (“HDR”) system, offer integrated data capabilities with much higherdata rates than that of second generation systems, which are capable ofoffering services such as streamed audio and video.

A cellular network consists of many geographic cells, each of which maycontain multiple sectors. Inside each cell, there is a base station. Auser typically communicates with the network through the sector thatprovides the best signal. When a mobile user changes location, the usermay communicate with the network through a different sector thatprovides the most reliable signal. Techniques for hand off in secondgeneration CDMA communication system are known in the art. However, CDMAdata communication systems, such as CDMA HDR, present new problems whena mobile unit selects a new sector.

One such problem occurs when a user switches among sectors too quickly.In a conventional CDMA cellular system, data traffic, which includesvoice, is routed to each sector that is actively communicating with amobile unit, possibly using multiple base stations. Consequently, allactive sectors in communication with a mobile unit send traffic to themobile unit. The redundancy in traffic was needed to meet the low-delayrequirements of voice data for handoff. This constraint is relaxed in adata network.

In a packet data network, users may tolerate short delays in the datatransmission. Since low delay is no longer a constraint on the system,reliability can be more efficiently achieved through re-transmissionrather than redundant transmission through all the active sectors allthe time in handoff scenario. Thus, in a conventional high rate packetdata cellular system, data traffic is typically routed through onesector that maximizes the forward link throughput. To accomplish thisrouting, the mobile monitors all the active sectors, among which theuser selects the best and informs the network of its selection. Such asystem exploits the channel dynamics in order to maximize the capacity.The selection of the transmitter to exploit local peaks in the shadowingprocess is a form of selection diversity. Thus, the selection of thebest serving sector is also referred to as site selection transmitdiversity (“SSTD”).

FIG. 1 illustrates a typical CDMA data communication system, such asCDMA HDR. Access network 100 contains several access points, of whichonly access points 110 and 130 are shown. A mobile unit, such as accessterminal 114, communicates with an access point, such as access point110, to connect to access network 100. In general, an access point, suchas access point 110, will have several sectors, such as sectors 116,118, and 120.

Since access terminal 114 generally communicates with one sector at atime, data going to access terminal 114 from access point 110 must berouted to the specific sector with which access terminal 114 iscommunicating.

However, a problem emerges when an access terminal is constantlyswitching among sectors. Suppose sector 116 has the strongest forwardlink signal at one instance such that access terminal 114 selects sector116 as the current serving sector. In a next instance, sector 132 ofaccess point 130 has the strongest forward link signal. Just momentslater, sector 116 again has the strongest forward link. It is possiblethat rapid switching between the two or more sectors can occur. Eachtime a switch occurs, data that was going to be sent to access terminal114, must be sent to the corresponding data queue for that sector.Further, the user cannot receive data before the data queue is ready.Such rapid transitions can create a significant amount of overhead forthe network, and outage for the user.

A second problem for selecting the best sector is related with thereverse link reliability. On the reverse link, access terminal 114 maysend channel state feedback information to the network to assist thenetwork in achieving the highest forward link throughput. In the highdata network system, access terminal 114 transmits a data rate controlsignal (“DRC”) to control the data rate on the forward link. Accessterminal 114 also sends an acknowledge signal (“ACK”) to the servingsector when it successfully receives a packet. Access terminal 114should select a new sector that has a reliable reverse link connectionwith access terminal 114. Otherwise, DRC and ACK information can belost, which reduces the throughput of the system. However, accessterminal 114 does not readily know the reliability of a reverse linkconnection. If access terminal 114 selects a sector with an unreliablereverse link, throughput can suffer due to retransmission.

Ideally, access terminal 114 should select a new sector so that itsthroughput on the forward link is maximized. Firstly, the site selectionshould avoid fast toggling. Secondly, the site selection shouldincorporate the impact of the reverse link reliability on forward linkthroughput. Thus, there is a need in the art for methods and systems forproperly selecting the best serving sector in a CDMA data communicationsystem.

SUMMARY

Embodiments disclosed herein address the above stated needs by usingsignal level and timing hysteresis and using a DRC reverse linkreliability information in site selection transmit diversity in a CDMAdata communication system.

The presently disclosed embodiments are directed to method and systemfor site selection transmit diversity in a CDMA data communicationsystem. According to one aspect of the present invention, signal levelsof the active sectors of an access terminal are compared with the signallevel of the current serving sector of the access terminal. Next, usinga signal level hysteresis, a delta credit is accumulated. When thereverse link reliability information is available, an accumulated totalcredit is authorized to produce an authorized accumulated total credit.Afterwards, the best serving sector is identified from a pool ofcandidate sectors based on the signal levels of the active sectors andthe authorized accumulated total credits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary CDMA data communication system accessnetwork comprising an access terminal an access points.

FIG. 2 illustrates an exemplary procedure for selecting a best servingsector.

FIG. 3 illustrates an exemplary procedure for comparing signal levels ofan active sector with the current serving sector.

FIG. 4 illustrates an exemplary procedure for accumulating delta creditsusing a signal level hysteresis.

FIG. 5 illustrates an exemplary procedure for credit authorization.

FIG. 6 illustrates an exemplary procedure for identifying a best servingsector.

FIG. 7 illustrates an exemplary procedure for selecting a best servingsector using RPC bits.

FIG. 8 illustrates exemplary systems for selecting a best servingsector.

DETAILED DESCRIPTION

The presently disclosed embodiments are directed to method and systemfor selecting the best serving sector to achieve site selection transmitdiversity in a CDMA data communication system. The following descriptioncontains specific information pertaining to the implementation of thepresent invention. One skilled in the art will recognize that thepresent invention may be implemented in a manner different from thatspecifically discussed in the present application. Moreover, some of thespecific details of the invention are not discussed in order not toobscure the invention. The specific details not described in the presentapplication are within the knowledge of a person of ordinary skill inthe art.

The drawings in the present application and their accompanying detaileddescription are directed to merely example embodiments of the invention.To maintain brevity, other embodiments of the invention which use theprinciples of the present invention are not specifically described inthe present application and are not specifically illustrated by thepresent drawings. The word “exemplary” is used exclusively herein tomean “serving as an example, instance, or illustration.” Any embodimentdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other embodiments.

FIG. 2 illustrates one embodiment of the invention. By way of example,the present embodiment of the invention operates in a CDMA communicationsystem. The general principles of CDMA communication systems, and inparticular the general principles for generation of spread spectrumsignals for transmission over a communication channel is described inU.S. Pat. No. 4,901,307 entitled “Spread Spectrum Multiple AccessCommunication System Using Satellite or Terrestrial Repeaters” andassigned to the assignee of the present invention. The disclosure inthat patent, i.e. U.S. Pat. No. 4,901,307, is hereby fully incorporatedby reference into the present application. Moreover, U.S. Pat. No.5,103,459 entitled “System and Method for Generating Signal Waveforms ina CDMA Cellular Telephone System” and assigned to the assignee of thepresent invention, discloses principles related to PN spreading, Walshcovering, and techniques to generate CDMA spread spectrum communicationsignals. The disclosure in that patent, i.e. U.S. Pat. No. 5,103,459, isalso hereby fully incorporated by reference into the presentapplication. Further, the present invention utilizes time multiplexingof data and various principles related to “high data rate” communicationsystems, and the present invention can be used in a “high data rate”communication systems, disclosed in U.S. patent application entitled“Method and Apparatus for High Rate Packet Data Transmission” Ser. No.08/963,386 filed on Nov. 3, 1997, and assigned to the assignee of thepresent invention. The disclosure in that patent application is alsohereby fully incorporated by reference into the present application.

Procedure 200 may operate in software at access terminal 114 and/oraccess point 110 for example. Procedure 200 contains steps fordetermining the best serving sector to achieve site selection transmitdiversity for access terminal 114. When operating in a CDMA HDR system,procedure 200 is called once each HDR slot while access terminal 114 isin the connected state, i.e. communicating with access network 100. Theprocedure begins in step 202 and proceeds to the next step. In step 204,access terminal 114 measures the forward link signal level from eachsector in the active set of pilots of access terminal 114, also referredto as the active sectors. Also, in step 204, the forward link signallevel for the current serving sector is measured.

In CDMA data communication systems, such as CDMA HDR, an access terminalcan receive traffic on the forward link transmission using eithervariable rate or fixed rate mode. Normally, when a reverse linktransmission is reliable, it is preferable for access terminal 114 toreceive data transmitted from the access terminal using variable datarates. Otherwise, when a reverse link transmission is not reliable, i.e.when DRC channel is not reliable for the sector with the strongestforward link signal, a fixed data rate mode may be used. In fixed ratemode, access terminal 114 receives traffic at a low, but steady, datarate to use for a plurality of slots. In one embodiment, the inventiondetermines the signal levels for both variable rate mode and fixed ratemode. In a later step, either variable rate mode or fixed rate mode canbe selected when a sector is chosen. Step 204 is presented in moredetail in FIG. 3.

In step 206, access terminal 114 determines the difference between theforward link signal levels from the active sectors and the forward linksignal levels from the current serving sector and then accumulates thedifferences for each slot to generate an accumulated total credit. Step206 is presented in more detail in FIG. 4.

In step 208, procedure 200 determines whether a new set of DRC lock bitshas been received. If a new set of DRC lock bits has been received, thenthe procedure continues to step 210. Otherwise, the procedure continuesto the end of the procedure at step 214.

In step 210, procedure 200 authorizes the accumulated total creditsbased on the DRC lock bits of the current serving sector and the activesectors. Since the DRC lock bits reveal whether the reverse link of thesectors are reliable, the DRC lock bits are utilized to generallyauthorize the credits of the sectors having a reliable reverse link andto discard the credits of the sectors having an unreliable reverse link.After step 210, the procedure continues to step 212. Step 210 ispresented in more detail in FIG. 5.

In step 212, procedure 200 uses the authorized accumulated total creditsfrom step 210 to identify the best serving sector. Step 212 is presentedin more detail in FIG. 6.

Procedure 300 of FIG. 3 expands on the signal level estimate step ofprocedure 200, namely step 204. Procedure 300 is called once per slotand is used to estimate the signal levels of the active sectors.Procedure 300 begins in step 302. In step 304, the procedure updates thepilot signal level estimates of the active sectors. Procedure 300 thendetermines a variable rate signal level and a fixed rate signal level.The variable rate signal level and fixed rate signal level arecalculated for each active sector. Both the variable rate and fixed ratesignal level estimates may be determined by a single pole infiniteimpulse response (“IIR”) filter.

As mentioned previously, the actual rate of data transmission in fixedrate state, also referred to as the “adjusted fixed rate signal level,”is generally set to be lower than the rate of transmission indicated bythe fixed rate signal level determined in step 304. To set the adjustedfixed rate signal level to below the fixed rate signal level, an offsetamount of energy in decibels (dB) is deducted from the fixed rate signallevel in step 306. In one embodiment, the offset amount of energy is 6dB. This adjustment to the fixed rate signal level, i.e. thedetermination of an adjusted fixed rate signal level, is performed forall active sectors.

After step 306, procedure 300 begins a series of comparisons between thesignal level of the current serving sector with that of all other activesectors. In each comparison, the variable rate and adjusted fixed ratesignal levels of the current serving sector are compared with thevariable rate and adjusted fixed rate signal levels of an active sector.

In step 308, the procedure takes the difference between the variablerate signal level of the current serving sector and the variable ratesignal level of an active sector. The difference is set to a difference,DiffVV. A separate DiffVV is stored for each comparison with a differentactive sector. The procedure continues at step 310.

In step 310, the procedure takes the difference between the adjustedfixed rate signal level of the current serving sector and the variablerate signal level of an active sector. The difference is set to adifference, DiffFV. A separate DiffFV is stored for each comparison witha different active sector. The procedure continues at step 312.

In step 312, the procedure takes the difference between the variablerate signal level of the current serving sector and the adjusted fixedrate signal level of an active sector. The difference is set to adifference, DiffVF. A separate DiffVF is stored for each comparison witha different active sector. The procedure continues at step 314.

In step 314, the procedure takes the difference between the adjustedfixed rate signal level of the current serving sector and the adjustedfixed rate signal level of an active sector. The difference is set to adifference, DiffFF. A separate DiffFF is stored for each comparison witha different active sector. The procedure ends at the end of procedure,step 316.

It is thus appreciated that each DiffXX value, i.e. DiffFF, DiffFV,DiffVF and DiffVV, determined in procedure 300 reflects the differencein signal level between the current serving sector and an active sector.As an example, DiffFV reflects the signal level difference between thecurrent serving sector in fixed mode and an active sector in variablemode.

Exemplary procedure 400 of FIG. 4 expands on the credit accumulationstep of procedure 200, namely step 206. Procedure 400, is performed onceevery slot. Procedure 400 accumulates the differences determined in FIG.3 according to a signal level hysteresis. In applying the signal levelhysteresis to the differences, each difference, DiffVV, DiffFV, DiffVF,and DiffFF, is compared to two thresholds. For example, in the presentembodiment, if a difference is between less than −3 dB, a correspondingdelta credit is incremented or “accumulated,” and if the difference isgreater than 0 dB the corresponding delta credit is decremented. Asignal level hysteresis ensures that only active sectors withsufficiently strong signal levels are identified for the purpose ofselecting a best serving sector.

Procedure 400 begins in step 402. In step 406, procedure 400 determinesif DiffVV is less than −3 dB. If DiffVV is less than −3 dB, thenprocedure 400 continues to step 408. Otherwise, procedure 400 continuesto step 412. In step 408, procedure 400 increments DeltaCreditVV by oneand continues to step 416.

In step 412, procedure 400 determines if DiffVV is greater than 0 dB. IfDiffVV is greater than 0 dB, then procedure 400 continues to step 414.Otherwise, procedure 400 continues to step 416. In step 414, procedure400 decrements DeltaCreditVV by one and continues to step 416.

In step 416, procedure 400 determines if DiffVF is less than −3 dB. IfDiffVF is less than −3 dB, then procedure 400 continues to step 418.Otherwise, procedure 400 continues to step 420. In step 418, procedure400 increments DeltaCreditVF by one and continues to step 424.

In step 420, procedure 400 determines if DiffVF is greater than 0 dB. IfDiffVF is greater than 0 dB, then procedure 400 continues to step 422.Otherwise, procedure 400 continues to step 424. In step 422, procedure400 decrements DeltaCreditVF by one and continues to step 424.

In step 424, procedure 400 determines if DiffFV is less than −3 dB. IfDiffFV is less than −3 dB, then procedure 400 continues to step 426.Otherwise, procedure 400 continues to step 428. In step 426, procedure400 increments DeltaCreditFV by one and continues to step 432.

In step 428, procedure 400 determines if DiffFV is greater than 0 dB. IfDiffFV is greater than 0 dB, then procedure 400 continues to step 430.Otherwise, procedure 400 continues to step 432. In step 430, procedure400 decrements DeltaCreditFV by one and continues to step 432.

In step 432, procedure 400 determines if DiffFF is less than −3 dB. IfDiffFF is less than −3 dB, then procedure 400 continues to step 434.Otherwise, procedure 400 continues to step 436. In step 434, procedure400 increments DeltaCreditFF by one and continues to step 440.

In step 436, procedure 400 determines if DiffFF is greater than 0 dB. IfDiffFF is greater than 0 dB, then procedure 400 continues to step 438.Otherwise, procedure 400 continues to step 440. In step 438, procedure400 decrements DeltaCreditFF by one and continues to step 440. In step440, procedure 400 ends.

Exemplary procedure 500 of FIG. 5 expands on the authorization step ofprocedure 200, namely step 210. Procedure 500 is performed after thedelta credits have been accumulated over L slots, where in oneembodiment L is set to 64. Procedure 500 authorizes the accumulatedtotal credits, i.e. DeltaCreditVV, DeltaCreditVF, DeltaCreditFV, andDeltaCreditFF, obtained in procedure 400. Procedure 500 authorizes thedelta credits for each sector based on the received reliabilityinformation for the reverse links of all the active sectors. Authorizingdelta credit can be implemented by adding or subtracting an appropriateamount to the accumulated total credits.

Procedure 500 begins in step 502. In step 504, procedure 500 determinesif the reverse link of the current serving sector is reliable. Thereverse link of the current serving sector is reliable if the DRC lockbit for the current serving sector is ‘1.’ If the DRC lock bit of thecurrent serving sector is ‘1,’ then procedure 500 continues to step 506.Otherwise, procedure 500 continues to step 512.

In step 506, procedure 500 determines if the reverse link of a sector jis reliable, i.e. whether the DRC lock bit of sector j is ‘1.’ If theDRC lock bit of sector j is ‘1,’ then procedure 500 continues to step508. Otherwise, procedure 500 continues to step 510. In step 508, sincesector j has been found to have a reliable reverse link, all the DeltaCredits accumulated for sector j are authorized. In other words, all theDelta Credits that were accumulated previously in procedure 400 forsector j based on the assumption that sector j has a reliable reverselink can now be accepted (i.e. authorized) because the DRC lock bit forsector j in fact validates the assumption. Thus, at step 508,DeltaCreditVV, DeltaCreditVF, DeltaCreditFV and DeltaCreditFF for sectorj are all authorized.

If it is instead determined at step 506 that the reverse link for sectorj is unreliable, i.e. that the DRC lock bit for sector j is ‘0,’ thenprocedure 500 proceeds to step 510. In step 510, only DeltaCreditVF andDeltaCreditFF are authorized, while DeltaCreditFV and DeltaCreditVV arerejected, i.e. they are not authorized.

If it is determined in step 504 that the reverse link of the currentserving sector is not reliable, procedure 500 continues at step 512. Instep 512, procedure 500 determines if the reverse link of the activesector j is reliable, i.e. DRC lock bit of sector j is ‘1.’ If the DRClock bit of sector j is ‘1,’ then procedure 500 continues to step 514.Otherwise, procedure 500 continues to step 516. In step 514, since thecurrent serving sector has an unreliable reverse link and sector j has areliable reverse link, the credits accumulated for sector j areauthorized. All the credits, i.e. DeltaCreditVV, DeltaCreditVF,DeltaCreditFV and DeltaCreditFF, can be authorized because the DRC lockbit validates the assumption that sector j has a reliable reverse link.

In step 516, since the reverse link for the current serving sector andsector j are both unreliable, only DeltaCreditVV, DeltaCreditVF andDeltaCreditFF are authorized. All the DeltaCreditFV for sector j arerejected.

Once the authorization or rejection of delta credits has been completedin steps 508, 510, 514 or 516, procedure 500 continues to step 518. Instep 518, the credits authorized for sector j in the preceding step,i.e. either step 508, 510, 514 or 516, are tabulated to generateCreditFF, CreditFV, CreditVF and CreditVV for sector j. It is noted thatany of CreditFF, CreditFV, CreditVF or CreditVV tabulated in step 518 isrepresented generally by CreditXX. Procedure 500 for the authorizationof the accumulated delta credits then ends at step 520.

The exemplary procedure of FIG. 6 expands of the identification step ofprocedure 200, namely step 212. Procedure 600 selects the best servingsector based on the authorized accumulated total credits from step 210,which was explained in FIG. 5 above.

Procedure 600 begins at step 602. In step 604, the credits, i.e.CreditXX, accumulated and authorized for a candidate sector, i.e. asector j of the active set, are compared to a threshold. The thresholdcan be, for example, set at 64. If CreditXX for the candidate sector isabove the threshold, procedure 600 continues to step 606 where valueFlagXX is set to ‘1’. Otherwise, procedure 600 continues to step 608where value FlagXX is set to ‘0’. Thus, value FlagXX indicates whetherthe tabulated credits for a candidate sector when compared to thecurrent serving sector are sufficiently high to support a switch fromthe current serving sector to the candidate factor. As an example, ifFlagFV for a candidate sector exceeds the threshold, it can beinterpreted to mean that the candidate sector in variable mode would bea better selection than the current serving sector in fixed mode. On theother hand, if FlagFV does not exceed the threshold, it can beinterpreted to mean that the better choice would be the current servingsector in fixed mode, rather than the candidate sector in variable mode.Steps 604, 606, and 608 are repeated for all candidate sectors and foreach variation of CreditXX, i.e. CreditFF, CreditFV, CreditVF andCreditVV, to generate a set of FlagFF, FlagFV, FlagVF and FlagVV foreach candidate sector. Procedure 600 then continues at step 610.

In step 610, the average variable rate signal for each of the candidatesectors is adjusted based on the received DRC lock bits. As has beendiscussed, the DRC lock bits indicate the reliability and quality of thereverse link; however they are not transmitted continually by thecandidate sectors in the active set. Rather, the DRC lock bits aretransmitted only intermittently, e.g. every 64 slots. Once they arereceived, the DRC lock bits can be utilized to determine the actualsignal level for a candidate sector in variable mode. The adjustedvariable rate signal level (“AdjVAR”) can be determined by deducting avalue based on the DRC lock bits from the average variable ratedetermined previously under the assumption that the reverse link wasreliable. Step 610 is performed once for each sector in the activesector set. In other words, an adjusted variable rate signal level,AdjVAR, is determined for each candidate sector.

Once the four FlagXX values and the AdjVAR have been determined for eachcandidate sector in the steps described above, procedure 600 continuesin step 612. In step 612, FlagVV and AdjVAR for a candidate sector areconcatenated to generate a single integer, SortKeyVariable1. Forexample, if FlagVV for the candidate sector is ‘x’ and the AdjVAR forthe candidate sector is ‘y,’ then SortKeyVariable1 for the candidatesector would be ‘xy.’ Using the same method, SortKeyVariable2 isdetermined by concatenating FlagFV and AdjVar, SortKeyFixed1 isdetermined by concatenating FlagVF and the adjusted fixed rate for thecandidate sector and SortkeyFixed2 is determined by concatenating FlagFFand the adjusted fixed rate for the candidate sector. Thus, for eachcandidate sector in the active set, step 612 generates four SortKeyvalues, namely SortKeyVariable1, SortKeyVariable2, SortKeyFixed1 andSortKeyFixed2.

The four SortKey values for the candidate sectors determined in step 612represent the relative improvement to be expected if a switch is madefrom the current serving sector to one candidate sector versus anothercandidate sector. For example, if the SortKeyVariable1 for a firstcandidate sector is greater than the SortKeyVariable1 for a secondsector, it can be concluded that, with the current serving sector invariable mode, a switch to the first candidate sector in variable modewould be a better switch than a switch to the second candidate sector invariable mode. As another example, if the SortKeyFixed2 for a firstcandidate sector is less than the SortKeyFixed2 for a second candidatesector, then it can be concluded that, with the current serving sectorin fixed mode, the better selection would be to switch to the secondcandidate sector in fixed mode than to switch to the first candidatesector in fixed mode.

Following the determination of the four SortKey values for each of thecandidate sectors in step 612, procedure 600 continues to step 614. Instep 614, the highest SortKey value for a switch to variable mode andthe highest SortKey value for a switch to fixed mode are identified. Thehighest SortKey value indicates the switch for both variable and fixedmode which will provide the best transmission quality and rate. Thus, inorder to identify the highest SortKey value for a switch to variablemode, the SortKeyVariable1 and SortKeyVariable2 values of all candidatesectors are compared against each other. The SortKeyVariable1 orSortKeyVariable2 having the highest value overall is identified as theHighestSortKeyVariable. Similarly, the SortKeyFixed1 and SortKeyFixed2values of all the candidate sectors are compared to identify theHighestSortKeyFixed.

Procedure 600 then continues to step 616 where theHighestSortKeyVariable is compared to the HighestSortKeyFixed. Whichevervalue is higher is selected as the preferred mode, i.e. the best servingsector. For example, if HighestSortKeyVariable is higher thanHighestSortKeyFixed, then the preferred mode would be variable mode andthe best serving sector would be the sector having theHighestSortKeyVariable. Procedure 600 then ends in step 620.

In an alternative embodiment, the procedure of FIG. 2 can determinereverse link reliability without the use of DRC lock bits. Exemplaryprocedure 700 illustrated in FIG. 7 uses reverse link power control bits(“RPC”) to determine the reliability of the reverse link in lieu of DRClock bits. Procedure 700 comprises several procedures that are found inprocedure 200. Procedure 700 begins in step 702. Step 704, is identicalto step 204 of procedure 200. It will be recalled that the details ofstep 204 were presented in procedure 300. Thus, procedure 300 detailsthe operation of step 704 as well.

In step 706, procedure 700 filters RPC bits of an active sector todetermine a corresponding reverse link reliability. RPC bits may befiltered using a single pole IIR to determine a mean value or averagevalue. If the average value of the RPC bits for the sector exceeds athreshold value, then it can be concluded that the sector has a reliablereverse link. Otherwise, the sector has an unreliable reverse link.

RPC bits are generally transmitted continuously by the active sectors,as opposed to being transmitted once per L slots in the case of DRC lockbits. Since, the reverse link reliability can be determined from the RPCbits, procedure 700 does not need to wait L slots to authorize theaccumulated delta credits. Instead, procedure 700 can select a new bestserving sector when an accumulated delta credit exceeds a signal levelthreshold as well as a duration threshold.

Also in step 706, procedure 700 applies a deduction to variable rate andfixed rate mode signal levels for each active sector. A deduction isapplied to variable rate signal level if the average or mean RPC valueis below a threshold. Otherwise, no deduction is applied. An exemplarydeduction for variable rate signal level can be 20 dB. A deduction isapplied to fixed rate signal level as well to ensure a lower rate forfixed rate transmission mode. An exemplary deduction for fixed ratesignal level can be 6 dB.

Next, step 708 of procedure 700 is similar to step 206 of procedure 200.Step 708 accumulates delta credits as per procedure 400 to produce anaccumulated total credits. The accumulated total credits of step 708 donot need to be authorized, since credit authorizing and referencing wereincluded in step 706. In step 710, procedure 700 identifies a new bestserving sector as per procedure 600, wherein procedure 700 substitutesaccumulated total credits for the authorized accumulated total creditsof procedure 600. After step 710, procedure 700 ends at step 712.

System 800 of FIG. 8a depicts exemplary procedure 200 in a system blockdiagram. Sector j signal level 802 and current serving sector signallevel 804 provide input to signal level estimator 806. Signal levelestimator 806 deducts an offset value from the fixed rate signal levelof the current serving sector and the sectors in the active set togenerate adjusted fixed rate signal levels for all sectors. Following,estimator 806 provides the signal level it measures, i.e. signal levels808, to signal level comparator 810. Comparator 810 determinesdifferences 812, DiffVV, DiffFV, DiffVF, and DiffFF as per procedure300. Differences 812 are provided as input to accumulator 814.Accumulator 814 applies a hysteresis during accumulation as perprocedure 400. Accumulator 814 provides accumulated total creditsDeltaCreditVV, DeltaCreditVF, DeltaCreditFV, and DeltaCreditVV to creditauthorization module 820. Current serving sector DRC lock bit 816 andsector j DRC lock bit 818 also provide input to credit authorizationmodule 820. After applying preferences and authorization to theaccumulated total credits as per procedure 500, credit authorizationmodule 820 provides authorized accumulated total credits 822 to newsector identification module 824. New sector identifier module 824selects the highest level among a pool of candidate sectors as peroperation of procedure 600. The pool of candidate sectors are formedfrom the active sectors and ordered according to procedures 500 and 600.New sector identifier 824 outputs new serving sector 826 andtransmission mode 828. Transmission mode 828 identifies the new servingsector transmission mode as fixed rate or variable rate.

System 850 of FIG. 8b depicts procedure 700 in a system block diagram.Sector j signal level 852 and current serving sector signal level 854provide input to signal level estimator 856. Signal level estimates 858and RPC bits of active sectors 862 provide input to RPC filter 860. Ifthe mean RPC exceeds a threshold, then a deduction is applied to thevariable rate signal level as per step 706 of procedure 700. Otherwise,no deduction is applied. RPC filter 860 supplies adjusted signal levels864 to comparator 866 and new sector identifier module 874. Comparator866 determines differences 868, DiffVV, DiffFV, DiffVF, and DiffFF asper procedure 300. Differences 868 are provided as input to accumulator870. Accumulator 870 applies a hysteresis during accumulation as perprocedure 400. Accumulator 870 provides accumulated total credits 872,DeltaCreditVV, DeltaCreditVF, DeltaCreditFV, and DeltaCreditVV to newsector identification module 874. New sector identifier module 874selects the sector with highest soft key among a pool of candidatesectors as per operation of procedure 600. Best sector identifier 874provides outputs best serving sector 876 and transmission mode 878.Transmission mode 878 identifies the new serving sector transmissionmode as fixed rate or variable rate.

The above procedures and system block diagrams overcome the problemsdiscussed. The above procedures and system block diagrams obtain theestimation of reverse link reliability from the reception of the DRCLock bits. In an alternative embodiment, the above procedures and systemblock diagrams may use a mean or average RPC value to estimate thereverse link reliability. In addition, by using a signal levelhysteresis and timing hysteresis, the procedures and system blockdiagrams overcome the problem of fast toggling. Thus, in the mannerdescribed above, the invention provides method and system for selectinga the best serving sector to achieve site selection transmit diversityin a CDMA data communication system. Those of skill in the art wouldunderstand that information and signals may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (“DSP”), an application specific integrated circuit (“ASIC”),a field programmable gate array (“FPGA”) or other programmable logicdevice, discrete gate or transistor logic, discrete hardware components,or any combination thereof designed to perform the functions describedherein. A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.The software module, also called a computer program in the presentapplication, may contain a number of source code or object code segmentsand may reside in any computer readable medium such as a RAM memory,flash memory, ROM memory, EPROM memory, EEPROM memory, registers, harddisk, a removable disk, a CD-ROM, a DVD-ROM or any other form ofcomputer readable medium known in the art. An exemplary computerreadable medium is coupled to the processor, where the processor canread information from, and write information to, the computer readablemedium. In the alternative, the computer readable medium may be integralto the processor. The processor and the computer readable medium mayreside in an ASIC. The ASIC may reside in a mobile unit, base stationtransceiver, or satellite transponder. In the alternative, the processorand the computer readable medium may reside as discrete components in auser terminal.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

We claim:
 1. A method for selecting a best serving sector in a CDMA communication system, said method comprising steps of: comparing a plurality of signal levels received from a plurality of active sectors with a signal level of a current serving sector to produce a difference; generating a delta credit for each of said plurality of active sectors based on said difference; accumulating a plurality of delta credits to produce an accumulated total credit; identifying said best serving sector from said accumulated total credit.
 2. The method of claim 1 wherein said plurality of signal levels received from said plurality of active sectors comprises a fixed rate signal level and a variable rate signal level.
 3. The method of claim 2 further comprising a step of adjusting said fixed rate signal level to produce an adjusted fixed rate signal level prior to said comparing step.
 4. The method of claim 3 further comprising a step of authorizing said plurality of delta credits prior to said identifying step.
 5. The method of claim 4 further comprising steps of: receiving a plurality of DRC lock bits; adjusting said variable rate signal to produce an adjusted variable rate signal level.
 6. The method of claim 5 further comprising steps of: determining a sector having a highest variable rate mode from said plurality of active sectors; determining a sector having a highest fixed rate mode from said plurality of active sectors.
 7. The method of claim 6 further comprising a step of determining a preferred mode.
 8. An apparatus for selecting a best serving sector in a CDMA communication system, said apparatus comprising: means for comparing a plurality of signal levels received from a plurality of active sectors with a signal level of a current serving sector to produce a difference; means for generating a delta credit for each of said plurality of active sectors based on said difference; means for accumulating a plurality of delta credits to produce an accumulated total credit; means for identifying said best serving sector from said accumulated total credit.
 9. The apparatus of claim 8 wherein said plurality of signal levels received from said plurality of active sectors comprises a fixed rate signal level and a variable rate signal level.
 10. The apparatus of claim 9 further comprising a means for of adjusting said fixed rate signal level to produce an adjusted fixed rate signal level.
 11. The apparatus of claim 10 further comprising a means for authorizing said plurality of delta credits.
 12. The apparatus of claim 11 further comprising: means for receiving a plurality of DRC lock bits; means for adjusting said variable rate signal to produce an adjusted variable rate signal level.
 13. The apparatus of claim 12 further comprising: means for determining a sector having a highest variable rate mode from said plurality of active sectors; means for determining a sector having a highest fixed rate mode from said plurality of active sectors.
 14. The apparatus of claim 13 further comprising a means for determining a preferred mode.
 15. A method for selecting a best serving sector in a CDMA communication system, said method comprising steps of: comparing a plurality of signal levels received from a plurality of active sectors with a signal level of a current serving sector to produce a difference, said plurality of signal levels comprising a fixed rate signal level and a variable rate signal level; generating a delta credit for each of said plurality of active sectors based on said difference; accumulating a plurality of said delta credits to produce an accumulated total credit; authorizing said accumulated total credit to produce a final credit total; identifying said best serving sector from said final credit total.
 16. The method of claim 15 further comprising a step of adjusting said fixed rate signal level to produce an adjusted fixed rate signal level prior to said comparing step.
 17. The method of claim 16 further comprising steps of: receiving a plurality of DRC lock bits; adjusting said variable rate signal to produce an adjusted variable rate signal level.
 18. The method of claim 17 further comprising steps of: determining a sector having a highest variable rate mode from said plurality of active sectors; determining a sector having a highest fixed rate mode from said plurality of active sectors.
 19. The method of claim 16 further comprising a step of determining a preferred mode.
 20. A method for selecting a best serving sector in a CDMA communication system, said method comprising steps of: determining a plurality of signal levels received from a plurality of active sectors, said plurality of signal levels comprising a fixed rate signal level and a variable rate signal level; adjusting said fixed rate signal level to produce an adjusted fixed rate signal level; comparing said plurality of signal levels received from said plurality of active sectors with a signal level of a current serving sector to produce a difference; generating a delta credit for each of said plurality of active sectors based on said difference; accumulating a plurality of said delta credits to produce an accumulated total credit; receiving a plurality of DRC lock bits for said plurality of active sectors; authorizing said accumulated total credit to produce a final credit total based on said plurality of DRC lock bits; identifying said best serving sector from said final credit total.
 21. The method of claim 20 further comprising steps of: determining a sector having a highest variable rate mode from said plurality of active sectors; determining a sector having a highest fixed rate mode from said plurality of active sectors.
 22. The method of claim 21 further comprising a step of determining a preferred mode.
 23. A computer readable medium including a computer program, said computer program for implementing a method for selecting a best serving sector in a CDMA communication system, said computer program comprising: a first code segment for comparing a plurality of signal levels received from a plurality of active sectors with a signal level of a current serving sector to produce a difference; a second code segment for generating a delta credit for each of said plurality of active sectors based on said difference; a third code segment for accumulating a plurality of delta credits to produce an accumulated total credit; a fourth code segment for identifying said best serving sector from said accumulated total credit.
 24. The computer readable medium of claim 23 wherein said plurality of signal levels received from said plurality of active sectors comprises a fixed rate signal level and a variable rate signal level.
 25. The computer readable medium of claim 24 further comprising a fifth code segment for adjusting said fixed rate signal level to produce an adjusted fixed rate signal level prior to said comparing said plurality of signal levels received from said plurality of active sectors with said signal level of said current serving sector to produce said difference.
 26. The computer readable medium of claim 25 further comprising a sixth code segment for authorizing said plurality of delta credits prior to said identifying said best serving sector from said accumulated total credit. 